Recent years have seen remarkable progress in the technical sophistication and downsizing of electronic devices, such as personal computers, cellular phones, and digital cameras. This progress has been accompanied by demands for the technical sophistication and downsizing of ceramics (key parts for electronic devices) for use in the electronics industry.
For example, because of its large band gap and excellent properties, such as electrical breakdown characteristic, heat resistance, and radiation resistance, silicon carbide has attracted attention as an electronic device material, for example, for compact and high-power semiconductors.
In addition, because of its excellent connectability to other compound semiconductors excellent in optical properties, silicon carbide has also attracted attention as an optical device material.
Silicon carbide is also excellent in mechanical properties, such as abrasion resistance, and thermal properties, such as heat resistance and thermal conductivity, while being significantly chemically stable and excellent in chemical resistance; silicon carbide thus has been used as a structural material, for example, for abrading agents, refractory materials, mechanical seals, and heat exchangers.
Known industrial methods for producing a silicon carbide powder include the Acheson process, silica reduction techniques, and silicon carbonization techniques. These methods, however, have submicron order limitations in mean particle size. Thus, dominant powders of silicon carbide have a mean particle size of submicron order or more.
There have been studies on new applications of silicon carbide powders in coating compositions for forming films, such as abrasion-resistant coating films, scratch-resistant coating films, heat-resistant coating films, and hardened coating films, as well as fillers for composite plating. In line with progress in nanotechnology, nanosized silicon carbide particles have also attracted great interest.
Examples of known methods for producing nanosized silicon carbide particles include a method using thermal plasma, which has a high temperature and high activity in a non-oxidizing atmosphere, and allows easy introduction of a fast cooling process (see Patent Literature 1).
The production method is useful for producing silicon carbide nanoparticles having a mean particle size of about 5 to 100 nm with excellent crystallinity, and the method can provide silicon carbide nanoparticles with a very small impurity content if a high-purity starting material is chosen.
Another known method is a silica precursor calcination method comprising the step of calcining a mixture of a silicon-containing substance, such as an organosilicon compound, silica sol, and silica hydrogel, a carbon-containing substance, such as phenolic resin, and a metal compound, such as lithium, for suppressing the growth of silicon carbide particles in a non-oxidizing atmosphere to thereby obtain silicon carbide particles (see Patent Literature 2).
This method is thought to provide an ultrafine silicon carbide powder containing no coarse particles.
To form a film, such as an abrasion-resistant coating film, a scratch-resistant coating film, a heat-resistant coating film, or a hardened coating film, using the nanosized silicon carbide powders obtained by the above-described methods, the following method is usable: applying a dispersion obtained by dispersing a nanosized silicon carbide powder in a solvent onto an object to thereby form a coating film; and heating the coated film to form a film of silicon carbide particles.
However, the nanosized silicon carbide particles obtained by the above-described methods are agglomerated with a stronger cohesive force between particles than oxide nanoparticles, such as alumina and silica. It is thus difficult to prepare a dispersion in which nanosized particles are homogeneously dispersed.
In addition, the particles of conventional nanosized silicon carbide powders are prone to aggregation. The powders, when dispersed in a solvent, become extremely viscous, thereby making it difficult to prepare a highly concentrated dispersion containing a fine silicon carbide powder in an amount of about 8 wt % or more.
Thus, it is only possible to prepare a low-concentrated dispersion for the above-described applications. The low concentration makes it difficult to thicken a coating film during formation. To thicken the coating film, a coating liquid must be applied and dried repeatedly. As noted, the nanosized silicon carbide particles have many drawbacks.
Examples of methods for increasing the dispersibility of silicon carbide particles include a method comprising oxidizing silicon carbide particles in an oxidizing atmosphere to form an oxidized surface layer so that the compatibility of the particles with the solution is increased (see Patent Literature 3).
However, this method is disadvantageous in that when silicon carbide particles are heated in an oxidizing atmosphere, melt-bonding occurs in the oxidized surface layer, leading to aggregation of the silicon carbide nanoparticles, which thereby makes it difficult to disperse the nanoparticles in a solution.